Hydraulic mining technique for recovering bitumen from subsurface tar sand deposits

ABSTRACT

A method for recovering bituminous petroleum from tar sand deposits by hydraulic mining. In one illustrative embodiment, an injection string having horizontally oriented nozzles on the lower end thereof and provided on the surface with means for rotating the injection string simultaneous with injecting fluids down the injection string. A separate flow path for recovery of the injected fluid with bitumen mixed therewith to the surface of the earth is also provided, which may be the annulus between an outer casing with the rotatable injection string in the center thereof. The fluid used in conjunction with this solution-mining technique is an aqueous solution of a polyphosphate wetting agent plus an alkalinity agent, heated to a temperature greater than the temperature of the tar sand deposit. The bituminous petroleum is separated from the phosphate solution by contacting the fluid with a hydrocarbon fluid such as diesel oil.

unlteu mates r'atent" H 1111 3,858,654 Walker 1 Jan. 7, 1975 [54] HYDRAULIC MINING TECHNIQUE FOR 3,782,472 l/l974 Siess. loo/303 RECOVERING BITUMEN FROM SUBSURFACE TAR SAND DEPOSITS Primary ExaminerErnest R. Purser Assistant Examiner.lack E. Ebel [75] Inventor: Clarence 0. Walker, R1chmond, Attorney Agent, or Firm Thomas H. whale); 0 G

Tex. Ries [73] Assignee: Texaco Inc., New York, NY. 22 Filed: June 18, 1973 [57] h ABSTRACT I A met od for recovering bituminous petro eum from [2]] Appl' 371204 tar sand deposits by hydraulic mining. In one illustrative embodiment, an injection string having horizon- [52] US. (:1 166/250, 166/272, 166/303 tally Oriented nozzles 9n the lower end thereof and 51 1m.c1 E2lb 43/24, E2lb 47/00 provided on the surface with means for rotating the [58] Field of Search 166/250, 267, 275, 272, injection string Simultaneous with injecting fluids 166/303 305 R down the injection string. A separate flow path for recovery of the injected fluid with bitumen mixed there- [56] Refer Cit d with to the surface of the earth is also provided, which UNITED STATES PATENTS may be the annulus between an outer casing with the rotatable injection string in the center thereof. The g fluid used in conjunction with this solution-mining 2882973 4/1959 j 166/305 R technique is an aqueous solution of a polyphosphate 3:19l:676 6/1965 Froning....... 166/275 Wetting agent P alkalinity 8 heated a 3,279,538 l0/l966 Doscher 166/272 Perawre greater than the temperature of the wt Sand 3,379,247 4/1968 Santovrian 166/27-2 deposit. The bituminous petroleum is separated from 3,396,792 8/1968 Muggee 166/303 the phosphate solution by contacting the fluid with :1 3,620,303 ll/l97l Halbert 166/272 hydrocarbon uch as diesel 3,687,197 8/l972 Redford 166/272 3,706,341 12/1972 Redford 166/272 25 ClaImS, l Drawmg Flgure HYDRAULIC MINING TECHNIQUE FOR RECOVERING BITUMEN FROM SUBSURF ACE TAR SAND DEPOSITS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to a method for recovering bitumen from subterranean tar sand deposits by hydraulic-mining, especially applicable to tar sand deposits not amenable to strip mining because the overburden thickness is too great.

2. Description of the Prior Art Petroleum is found in subterranean formations or reservoirs in which it has accumulated, and recovery of conventional petroleum is achieved by penetrating these reservoirs with wells and permitting the fluid to flow to the surface as a result of natural pressure existing in the reservoir, or by pumping the fluid to the surface in some instances where insufficient natural pressure exists to force it to flow to the surface. There are many petroleum-containing reservoirs which contain hydrocarbon which is too viscous to be pumped from the reservoir under normal circumstances. When such reservoirs are encountered, production is possible only by means of some process of supplemental recovery, commonly referred to as secondary or teritary recovery, in which energy is supplied to the reservoir to force the petroleum to move, or heat and/or a solvent is supplied to the reservoir to reduce the viscosity of the petroleum.

The most extreme example of formations which contain petroleum too viscous to recover by conventional means are the so-called tar sands or bitumen sands, such as those located in the Western United States, Western Canada, and Venezuela. These formations are known to contain huge reserves of bituminous petroleum, but the bituminous petroleum contained therein is too viscous to be recoverable by conventional techniques.

The present state of the art for the recovery of bitumen from tar sand deposits can be generally classified as strip mining or in situ separation. Strip mining requires removal of the overburden by mechanical means and the mixture of bitumen and sand that constitutes the tar sand deposit is then similarly removed by mechanical means and transported to a surface processing plant for separation of bitumen and sand. In situ separation processes make use of techniques for separating the bitumen from the sand within the tar sand deposit itself, so the bitumen in some modified form may be transported to the surface with the sand left in the tar sand deposit. Techniques presently employed for insitu separation may be classified as thermal or emulsiflcation processes. The thermal techniques include in situ combustion, (fire flooding), and steam flooding. Emulsification processes also involve the use of steam plus some additional chemical to promote emulsiflcation of the high viscosity bitumen so that it may be transported to the surface where the emulsion is resolved into bitumen and water. Although many in situ separation techniques have been proposed in the prior art, none have been both economically and technically successful.

Most known in situ processes involve injection of fluid under fairly high pressures. Injection of high pressure fluid can be conducted safely only if the formation overburden thickness is great enough to contain the pressure without rupturing. Strip mining of a tar sand deposit is economically feasible only if the ratio of overburden thickness to tar sand deposit thickness is around one or less. Many deposits exist wherein the overburden thickness is too great to permit exploitation by strip mining, and not great enough to contain high pressure fluids.

In view of the foregoing, it can be appreciated that there is a substantial, unfulfilled need for a method for recovery of bituminous material from tar sand deposits, particularly those deposits which are not suitable for strip mining or for in situ recovery processes involving injection of a high pressure fluid.

BRIEF DESCRIPTION OF THE DRAWING The attached drawing illustrates in cross-sectional view both the surface and subsurface equipment and completion for application of my process for hydraulicmining of tar sand deposits.

SUMMARY OF THE INVENTION I have discovered, and this constitutes my invention, that bitumen may be recovered from subterranean tar sand deposits by a hydraulic-mining technique wherein the tar sand is contacted by a hot, aqueous polyphosphate solution containing an alkaline substance such caustic or sodium hydroxide. One means for accomplishing this process employs a rotating injection string equipped on its lower end with jet nozzles so that the fluid may be directed as a jet stream against the tar sand deposit face. A separate communication path to the surface of the earth facilitates movement of the injected fluid with bitumen dispersed therein to the surface for further processing. The injection string is constructed so as to permit its rotation as the fluid is being injected down the injection string and out through the jet nozzles on the end thereof, so that a stream of fluid sweeps the tar sand deposits. Polyphosphate wetting agents suitable for use in my invention include sodium trimetaphosphate and sodium tetrametaphosphate. The bituminous petroleum is then separated from the polyphosphate solution by contacting same with a hydrocarbon fluid such as diesel oil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS My invention can be best understood by a reference to the attached drawing, in which a tar sand deposit I is located at too great a depth for economical strip mining and not deep enough to permit thermal recovery by high pressure fluid injection. A combination injection production well 2 is drilled to the bottom of the tar sand deposit, and casing 3 is set to the top of the formation. A separate injection string 4 is run inside the casing 3, to a point near the bottom of the tar sand deposit. The injection string 4 is equipped with nozzles 5 near the bottom thereof, and the completion equipment on the surface includes means for rotating the injection string as fluid is pumped down the string. This is illustratively embodied in the drawing by motor 6 coupled to injection string 4, and sealing devices 7 and 8 which insure a fluid-tight connection between the rotating and nonrotating portions of the surface equipment. A pump 9 pumps the hot alkaline polyphosphate solution down the injection string with sufficient pressure to provide the jetting action and insure contact with the tar sand essential to the proper functioning of my process.

A separate flow channel for returning a mixture of solvent and bitumen to the surface of the earth is also provided. This may be in the form of a separate well, or the annular space 10 between casing 3 and injection string 4 in the drawing may be utilized for this purpose. On the surface, fluid conduit 11 is connected to annular space 10 on one end and on the other end to separation tank 12. Separation tank 12 which may be equipped with a mixing device and heater has connected thereto flow line 13 which is connected to the output of pump 14 which takes suction from hydrocarbon treating fluid tank 15. Polyphosphate solution make up tank 16 is connected via pump 9 to the top of injection string 1.

In operation, the alkaline aqueous polyphosphate solution is pumped from supply tank 16. It is not necessary to inject the hot polyphosphate solution with as high pressure at the start as will be required later in the process since the solution jet will only have to travel a relatively short distance before contacting; the face of the tar sand deposit. The hot polyphosphate solution is injected via string 4 under pressure and out through nozzle against the face of the tar sand deposit. The bitumen is contacted by the hot polyphosphate solution, and aided by the high velocity of the fluid jet 17, bitumen coated sand is readily dislodged from the tar sand formation. Emulsiflcation does not occur, and much of the sand settles to the bottom of the cavity in the tar sand deposit. Some sand, particularly the fine grain sand, may be produced to the surface.

As bitumen and some sand are removed from the tar sand deposit, a cavity is created adjacent to the nozzles on injection string 4, and the size of this cavity increases with time as is illustrated on the drawing with dashed time contour lines which are designated T T and T As the cavity increases, it is necessary to increase the injection pressure so that the jet stream 17 will reach to the cavity walls with sufficient veolcity to dislodge bitumen coated sand.

Throughout the process of my invention, a mixture of bitumen and hot polyphosphate solution, with a minor amount of sand suspended therein, flows back to the surface of the earth via the communication path provided therefor by annulus in the example illustrated in the drawing. The produced bitumen-polyphosphate solution mixture passes via flow line 11 into separation tank 12. Hydrocarbon treating fluid such as diesel oil contained in tank is pumped via pump 14 into tank 12 to cause separation of bitumen and sand. Sand settles to the bottom and is removed mechanically via line 18. Bitumen and hydrocarbon treating fluid separate into a top phase and is removed by line 19 and then to surface processing equipment. Hot polyphosphate solution constitutes the third phase, passing via line 20 back to tank 16 where it can be recycled into the injection string 4.

The filud injected in the form of a jet and preferably a rotating jet, is an aqueous alkaline solution of a polyphosphate such as tetrasodium pyrophosphate Na P- 0,, sodium acid pyrophosphate Na,H,P,O or pentasodium tripolyphosphate, plus an alkalinity agent such as caustic soda (sodium hydroxide, NaOH).

Polyphosphates are inorganic salts whose anions contain P0 units linked by sharing oxygens with other tetrahedra. The polyphosphates may be in chain form,

4 e.g., [P 0 1 1 or [P 0 1 1 or in a ring form IP O P and [P 0 1 1 triand tetra-metaphosphates respectively.

Polyphosphates are hydrophilic surface active agents as opposed to organic wetting agents which may be considered hydrophobic surface active agents. There are no oleophilic moieties present in inorganic polyphosphates so they are not surfactants in the sense of alkyl or alkylaryl sulfates, sulfonates or phosphates. The prior art teaches the use of polyphosphates as builders in house-hold detergents, and that polyphosphates function as defloculants in drilling fluids and other suspensions, and for the purpose of forming soluble complexes.

Any water soluble salt of pyrophosphoric acid, H P O may be used in the process of my invention. For example, the sodium, potassium or lithium salt of pyrophosphoric acid may be used. There are usually more than one salt for each cation known. For example, the sodium salts are tetrasodium pyrophosphate, Na P O trisodium hydrogen pyrophosphate, Na HP O disodium pyrophosphate (known commercially as sodium acid pyrophosphate) Na H P O and sodium trihydrogen pyrophosphate, NaH P O Other sodium polyphosphates such as pentasodium tripolyphosphate, Na P O are also known and may be used in my process. Ring polyphosphates such as sodium trimetaphosphate Na (PO and sodium tetrametaphosphate Na (PO may also be used.

Generally the preferred materials are the water soluble salts of pyrophosphoric acid, and especially sodium acid pyrophosphate or tetrasodium pyrophosphate.

The concentration of polyphosphate in the aqueous solution may be quite dilute, as low as 0.052-2 percent. Although concentrations greater than this may be used, there is no particular advantage in using larger concentrations, and while the cost of the material is low, economics of the process are optimized by using the lowest effective concentration.

Ordinarily an alkalinity agent will be added to the polyphosphate solution. Care should be taken to insure that the cation associated with the alkalinity agent does not form an insoluble salt with the particular polyphosphate being used. Generally sodium hydroxide is the preferred material, although potassium hydroxide. lithium hydroxide or ammonium hydroxide may be used also.

Sufficient alkalinity agent should be added to the solution to bring the pH thereof to a value above 10 and preferably above l2. Generally 0.1-5.0 percent by weight will be sufficient. One convenient means for maintaining the desired pH is to add approximately equal amounts of the alkalinity agent and the polyphosphate.

Heating the aqueous alkaline polyphosphate to a temperature in excess of the natural formation temperature will increase the effectiveness of this hydraulic solution mining technique. The solution may be heated on the surface just prior to injecting it into the injection means. The preferred operating temperature is from about F. to about 200F.

One attractive feature of the process of my invention is the fact that bitumen is not emulsified on being contacted with the hot aqueous alkaline polyphosphate solution. Bitumen is removed effectively from the sand but remains in a separate phase which will form a discrete layer distinct from the aqueous solution if agitation is stopped. The bitumen may be pumped to the surface leaving most of the polyphosphate solution in the cavity formed in the subterranean tar sand formation if desired.

Much of the sand from which the bitumen is removed settles to the bottom of the cavity, eliminating the need for handling large amounts of sand on the surface. Some sand may be carried to the surface along with the bitumen, but the bulk of the sand will remain in the cavity in the subterranean formation.

As the distance from the injection point to the face of the cavity increases, the injection pressure should be increased so that the fluid jet will continue to contact the cavity wall. The injection pressure should not be raised to the point where fracture of the overburden will result. As a general rule, the pressure in pounds per square inch should not exceed the overburden thickness in feet. Eventually the cavity size will become so great that the fluid stream no longer reaches the cavity wall. Some additional bitumen recovery may be possible by pumping all fluid from the cavity so the injected fluid can reach greater distances from the injection nozzle. The end point of the process is easily detected on the surface by a decline in the rate of bitumen production.

Separation of bitumen and aqueous solution occurs quickly when agitation is stopped. Separation speed and efficiency is'enhanced by contacting the mixture with a hydrocarbon separation fluid, such as diesel. The separation fluid may be added continuously or intermittenly, on the surface or in the tar sand formation. Field Example A tar sand deposit is to be exploited and it is determined that the thickness of the tar sand deposit is 50 feet and the thickness of the overburden is 125 feet. Since the ratio of overburden thickness to tar sand deposit thickness is greater than I, strip mining is ruled out on economic basis. A well is drilled to the bottom of the deposit and a casing is set to the top portion of the tar sand deposit. A tubing string is inserted into the casing, the bottom portion of the tubing string being equipped with two horizontally oriented jet nozzles so that fluids pumped into the tubing string will exit through these nozzles in a generally horizontal direction with considerable velocity. The injection string is equipped on the surface with means for rotating the injection string by an electric motor, and sealing devices to establish a liquid tight seal between the rotating and nonrotating members are also provided. An aqueous solution of 0.3 percent sodium acid pyrophosphate and 0.3 percent sodium hydroxide heated to a temperature of 150 F. is injected into the injection means by a pump located on the surface. Initially the injection pressure is approximately 50 pounds per square inch, since the jet emerging from the nozzles must flow only a short distance before it impinges against the tar sand deposits. The mixture of bitumen from the tar sand and the polyphosphate solution is produced to the surface via the annular conducting space between the rotating injection string and the casing. The produced mixture is passed to a separation tank located on the surface, and diesel oil is added to the mixture to aid in rapid separation.

As the cavity increases, the solvent jet streams from the nozzles on the end of the injection string must travel further away from the injection point before contacting the wall of the cavity in the tar sand deposit, and

so the injection pressure must be increased. The need for an increase in injection pressure is determined by monitoring the concentration of bitumen in the bitumen-polyphosphate solution mixture being produced to the surface of the earth. A decrease in the concentration of bitumen in the produced fluid indicates that the jets of polyphosphate solution are not moving sufficiently far away from the nozzles to contact virgin tar sand, and so the injection pressure must be increased. The injection pressure is increased slowly since it is not desired to create a fracture between the pressurized tar sand formation and the surface of the earth which would establish an undesired return communicating path through the overburden to the surface. By increasing the injection pressure in small increments, e.g., 5 or 10 psi at a time, the injected solvent stream may be made to continually contact the outer cavity walls within the tar sand deposit. The injection pressure expressed in pounds per square inch is not allowed to exceed the overburden thickness expressed in feet. This process is continued until a substantial decrease in bitumen content of the produced bitumen-containing fluid is evidenced, and an increase in injection pressure up to psi fails to cause a corresponding increase in the bitumen content of the produced fluid. This indicates that the maximum range of the polyphosphate fluid jet within the cavity has been reached and no additional bitumen can be recovered by this technique from the cavity.

After it has been determined that the solution-mining process has been extended as far into the tar sand deposit as possible, the polyphosphate solution remaining within the cavity may be recovered for reuse in adjacent areas of the deposit by pumping the fluid from the cavity.

EXPERIMENTAL In order to verify the operability of my invention and further to determine the optimum values for the con-' trollable parameters, the following experimental work was performed.

One hundred gram samples of tar sand obtained from the Athabasca area in Alberta, Canada were compressed into cylindrical cakes at 6,000 pounds per square inch pressure. The samples were placed in an open container filled with the various test solutions. A motor driven mixer blade was positioned in the solution approximately 1 k inches from the face of the tar sand cake and rotated. The extent of disintegration of the tar sand at the end of 15 minutes was observed and this observation is reported below. In all cases the temperature was maintained at l25l 30F.

TABLE I Cake Condition at Fluid End of l5 Minutes tap water unchanged water +03% Sodium Acid Pyro Phosphate unchanged 50% disintegrated [00% disintegrated |00% disintegrated slight disintegration After disintegration of the tar sand samples, the mixtures were allowed to settle. Clean sand settled to the bottom and distinct layers of bitumen and polyphosphate solution formed. Only a slight amount of hydrofrom the true spirit and scope of my invention. Further more, it is not my intention to be bound by any particular explanation of the mechanism responsible for the benefits resulting from application of the process of my invention. It is my intention that my invention be limited only by such restrictions and limitations as may be imposed by the appended claims.

I claim:

1. A method for recovering bitumen from a subterranean tar sand deposit having injection means and production means comprising:

contacting the tar sand deposits with an aqueous solution comprising an inorganic polyphosphate substance plus an alkalinity agent, the temperature of said solution being at least as great as the temperature of the tar sand. deposit.

2. A method as recited in claim 1 wherein the aqueous polyphosphate solution is made to contact the tar sand by jetting action.

3. A method as recited in claim 1 wherein the injection means is at least one well contining an injection string for introducing the polyphosphate solution into the tar sand deposit.

4. A method as recited in claim 3 wherein the injection string is equipped on its lower end with at least one horizontally oriented nozzle.

5. A method as recited in claim 3 wherein surface means are provided for rotating the injection string and said string is rotated while the polyphosphate solution is being injected.

6. A method as recited in claim 4 wherein the annular space between the injection string and well casing is used for production means.

7. A method as recited in claim 1 wherein the polyphosphate is a water soluble salt of pyrophosphoric acid.

8. A method as recited in claim 7 wherein the polyphosphate is a sodium acid pyrophosphate.

9. A method as recited in claim 7 wherein the polyphosphate is sodium trihydrogen pyrophosphate.

10. A method as recited in claim 7 wherein the polyphosphate is trisodium hydrogen pyrophosphate.

11. A method as recited in claim 7 wherein the polyphosphate is tetrasodium pyrophosphate.

12. A method as recited in claim 7 wherein the polyphosphate is a potassium salt of pyrophosphoric acid.

13. A method as recited in claim 7 wherein the polyphosphate is the lithium salt of pyrophosphoric acid.

14. A method as recited in claim 1 wherein the polyphosphate is a water soluble tripolyphosphate salt.

15. A method as recited in claim 14 wherein the phosphate is pentasodium tripolyphosphate.

16. A method as recited in claim 1 wherein the concentration of polyphosphate is at least 0.10 percent by weight.

17. A method as recited in claim 1 wherein the concentration of polyphosphate is from 0.10 percent to about 5.0 percent by weight.

18. A method as recited in claim 1 comprising the additional step of contacting the mixture of bitumen and polyphosphate solution with a hydrocarbon fluid to promote separation of bitumen from the aqueous polyphosphate solution.

19. A method as recited in claim 18 wherein the hydrocarbon fluid is diesel oil.

20. A method as recited in claim 1 wherein the alkalinity agent is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide.

21. A method as recited in claim 20 wherein the alkalinity agent is sodium hydroxide.

22. A method as recited in claim 1 wherein the concentration of alkalinity agent is from about 0.10 to about 5.0 percent by weight.

23. A method as recited in claim 1 wherein the polyphosphate solution is injected via the injection means and the injection pressure is increased gradually with time to a value in pounds per square inch not exceeding the over-burden thickness in feet.

24. A method as recited in claim 1 wherein the production rate of bitumen is monitored continually and the injection pressure is increased in response to a decrease in bitumen production rate.

25. A method as recited in claim 1 wherein the polyphosphate solution is heated to a temperature of from around to about 200 F. 

1. A METHOD FOR RECOVERING BITUMEN FROM A SUBSTERRANEAN TAR SAND DEPOSIT HAVING INJECTION MEANS AND PRODUCTION MEANS COMPRISING: CONTACTING THE TAR SAND DEPOSITS WITH AN AQUEOUS SOLUTION COMPRISING AN INORGANIC POLYPHOSPHATE SUBSTANCE PLUS AN ALKALINITY AGENT, THE TEMPERATURE OF SAID SOLUTION BEING AT LEAST AS GREAT AS THE TEMPERATURE OF THE TAR SAND DEPOSIT.
 2. A method as recited in claim 1 wherein the aqueous polyphosphate solution is made to contact the tar sand by jetting action.
 3. A method as recited in claim 1 wherein the injection means is at least one well contining an injection string for introducing the polyphosphate solution into the tar sand deposit.
 4. A method as recited in claim 3 wherein the injection string is eqUipped on its lower end with at least one horizontally oriented nozzle.
 5. A method as recited in claim 3 wherein surface means are provided for rotating the injection string and said string is rotated while the polyphosphate solution is being injected.
 6. A method as recited in claim 4 wherein the annular space between the injection string and well casing is used for production means.
 7. A method as recited in claim 1 wherein the polyphosphate is a water soluble salt of pyrophosphoric acid.
 8. A method as recited in claim 7 wherein the polyphosphate is a sodium acid pyrophosphate.
 9. A method as recited in claim 7 wherein the polyphosphate is sodium trihydrogen pyrophosphate.
 10. A method as recited in claim 7 wherein the polyphosphate is trisodium hydrogen pyrophosphate.
 11. A method as recited in claim 7 wherein the polyphosphate is tetrasodium pyrophosphate.
 12. A method as recited in claim 7 wherein the polyphosphate is a potassium salt of pyrophosphoric acid.
 13. A method as recited in claim 7 wherein the polyphosphate is the lithium salt of pyrophosphoric acid.
 14. A method as recited in claim 1 wherein the polyphosphate is a water soluble tripolyphosphate salt.
 15. A method as recited in claim 14 wherein the phosphate is pentasodium tripolyphosphate.
 16. A method as recited in claim 1 wherein the concentration of polyphosphate is at least 0.10 percent by weight.
 17. A method as recited in claim 1 wherein the concentration of polyphosphate is from 0.10 percent to about 5.0 percent by weight.
 18. A method as recited in claim 1 comprising the additional step of contacting the mixture of bitumen and polyphosphate solution with a hydrocarbon fluid to promote separation of bitumen from the aqueous polyphosphate solution.
 19. A method as recited in claim 18 wherein the hydrocarbon fluid is diesel oil.
 20. A method as recited in claim 1 wherein the alkalinity agent is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide.
 21. A method as recited in claim 20 wherein the alkalinity agent is sodium hydroxide.
 22. A method as recited in claim 1 wherein the concentration of alkalinity agent is from about 0.10 to about 5.0 percent by weight.
 23. A method as recited in claim 1 wherein the polyphosphate solution is injected via the injection means and the injection pressure is increased gradually with time to a value in pounds per square inch not exceeding the over-burden thickness in feet.
 24. A method as recited in claim 1 wherein the production rate of bitumen is monitored continually and the injection pressure is increased in response to a decrease in bitumen production rate.
 25. A method as recited in claim 1 wherein the polyphosphate solution is heated to a temperature of from around 100* to about 200* F. 